The standards of CDs, CD-ROMs and the like which are conventional optical disks are disk diameter=120 mm, disk thickness=1.2 mm, and structure=a single plate utilizing one surface. On the contrary, the standards of digital video disks (DVDs) are disk diameter=120 mm, disk thickness=1.2 mm, and structure=both surfaces stuck. Accordingly, the thickness of each disk substrate is 0.6 mm. As materials of these disk substrates, polycarbonates are much used in recent years, as having high transparency, good heat resistance, good impact resistance and good water resistance.
However, thin disk substrates, of which the thickness is reduced from 1.2 mm to 0.6 mm as so mentioned in the above, are often problematic in that they will be cracked or broken during molding and that their birefringence may increase. In order to prevent the disks from being cracked during molding, the molecular weight of the polycarbonate which Ls the substrate material can be increased, but in this case, the birefringence of the disks further increases. On the other hand, in order to reduce the birefringence, it has been attempted to mold the digital video disk substrates by the use of techniques such as ultrasonic molding, multi-stage compression molding and high-speed filling molding. Even by these techniques, however, the reduction in the birefringence is not sufficient, and what is worse, the breakage of the disks more often occurs as compared with usual molding techniques.
Under such circumstances, the present invention has been attained, and an object of the invention is to provide a digital video disk substrate which is cracked little during molding and which exhibits small birefringence and excellent transcriptional ability.